The present invention relates to an electrode unit for electrically heating underground hydrocarbon resources and a process for producing the same. More particularly, the invention relates to an electrode unit for electrically heating an underground hydrocarbon containing stratum so that hydrocarbons of high viscosity and low flowability in that stratum are rendered sufficiently mobile to be easily recovered through a well. The invention also relates to a process for producing such an electrode unit.
Typical examples of underground hydrocarbons having high viscosity and low flowability are the bitumen present in oil sands or tar sands, and the kerogen present in oil shale.
Intensive studies have been made on the economic use of oil sands. Two methods are the subjects of current studies on the heating of underground oil formations: one involves injecting hot water or high-pressure steam into an underground oil formation through a steel casing, and the other method uses the Joule heat generated by applying an electric current between two electrodes spaced in the oil formation. The first method can be implemented with simple equipment, but it achieves only low efficiency. On the other hand, the second method has a theoretically very high efficiency (which has been verified by experiment), but it requires a highly sophisticated apparatus. The present invention relates in one aspect to an electrode unit for use in the second method.
Electric heating alone is unable to recover oil sands from the ground. In actual operation, electric current is applied to a pair of tubular electrodes attached to the bottom of casings, and after the viscosity of the oil is reduced, high-pressure steam is injected into one casing so as to pump the oil up through the other casing.
For a better understanding of the present invention, the characteristics required of an electrode unit used in the recovery of oil sands are described below, together with the state in which oil sands occur naturally and the method of their recovery.
Proved oil-sand deposits have been found in Canada, the United States and Venezuela. The oil in the oil-sand formations is present on the surface of the sand and between sand particles, often together with salt water. This oil is extremely viscous and does not flow in the naturally occurring state. Oil sand deposits are sometimes exposed in valleys or on river banks, but in almost all cases, they lie in a stratum several tens of meters thick and 200 to 500 meters below the surface of the ground. Oil sands could be excavated and the oil separated on the ground surface, but this is not a recommended practice, not only from an economical viewpoint, but also from ecological aspects. The oil alone must be extracted from the ground. The recovery of oil from a shallow deposit involves the danger of cave-in of the earth's crust, and thus it is generally recommended that the oil be extracted from strata lying at least 300 meters below the surface of the ground.
The biggest problem with the method of heating an oil-sand deposit by applying an electric current between electrodes is that the oil-sand deposit has a higher electrical resistance than the overlying geological formation. Although generalization is difficult because of variations among location and geological conditions, the oil-sand deposits have an average electrical resistivity of 100 ohm-m whereas the overlying formation has a resistivity of 10 ohm-m. If a current is impressed between two electrode units each consisting of an electrode buried in the oil-sand deposit and connected to a steel casing, the greater part of the current flows through the geological formation lying above the oil-sand deposit. In order to avoid this phenomenon, the surface of the casing in the layer above the oil-sand deposit must be covered with an insulator coat, or alternatively, each electrode must be insulated from the casing.
One aspect of the present invention concerns an improvement of the second approach. Such an electrode unit is shown schematically in FIG. 1, wherein steel casings 1, 11 have electrodes 3, 13 connected thereto through insulating members 2, 12. The electrodes 3, 13 are further connected to a power supply 5 on the ground through cables 4, 14. When a voltage is applied from the power source 5 to the electrodes 3, 13 in an oil sand stratum 6 through the cables 4, 14, a current 7 flows through the oil-sand stratum 6 and Joule heat is generated in an amount that increases with the electrical resistance of the oil-sand stratum 6. This Joule heat provides energy for heating the oil-sand stratum 6. Part of the current 7 flows not only through a formation 9 above the oil-sand stratum, but also through an underlying formation 10. This leakage current can be reduced by the insulator members 2, 12 disposed between the casings 1, 11 and electrodes 3, 13. When the temperature of the oil-sand stratum 6 has reached a predetermined value, the current is discontinued and hot water or high-pressure steam is injected into one of the two casings of the electrodes, for example, casing 1, from its top. The injected hot water or steam passes through the oil-sand stratum 6 and pushes the oil up through the other casing 11. In order to ensure smooth outflow of the hot water or high-pressure steam, a number of pores are usually provided in the electrodes 3, 13.
The electrode unit is usually fed with a sodium chloride solution through a separate pipe (not shown) in order to reduce the contact resistance between the electrodes 3, 13 and the oil-sand stratum 6. Partitions (not shown) are provided above the electrodes 3, 13 for the purpose of isolating the sodium chloride solution from the casings 1, 11, and the space above the partitions is filled with an insulating fluid.
The electrode unit described above must satisfy various requirements. First, it should not break during installation work. After the installation, the unit should be strong enough to withstand the pressure of the surrounding soil. Even when the temperature of the unit is increased as a result of the impression of an electric current (a particularly great temperature increase occurs in the neighborhood of each electrode because of high current density), the unit should be able to withstand the static pressure of the fluid in it without deformation or rupture. Finally, the unit should not burst or cause a leak during injection of hot water or high-pressure steam. As a guide, a electrode unit buried 500 meters below the surface of the ground is subjected to a pressure of 50 kg/cm.sup.2 if the fluid with which the unit is filled has specific gravity of unity and, additionally, steam having a pressure of 50 kg/cm.sup.2 and a temperature as hot as 265.degree. C. can be passed therethrough.
The top of the insulating member 2 (12) is connected to the casing 1 (11) and the bottom is connected to the electrode 3 (13) so that the subjection of the insulator members 2, 12 to the pulling action of the electrodes is maintained. Since the electrodes are heated to 250.degree. to 300.degree. C., the insulator members 2, 12 are required to withstand not only the pulling action of the electrodes but also the high temperatures to which they are heated. When the insulator members are buried under the ground, usually a few hundred meters deep, they are installed as an assembly with the electrodes (3, 13) and casings (2, 12), and therefore it is practically impossible to prevent the insulators from contacting or colliding with the walls of the holes down which they are being pushed therethrough. Since the complete electrode unit is quite heavy, the slightest contact with the walls of the holes will cause a great mechanical impact on the insulators. Therefore, the insulator members 2, 12 are also required to have sufficient strength to safely withstand this mechanical impact.
Further, the present invention relates to an insulated metal cylinder and a process for producing the same. More particularly, the invention relates to a long insulated metal cylinder that has an insulator coat formed on the outer surface of a metal pipe or rod and which can be used in an temperature range of room temperature up to 300.degree. C. without spalling or breakage of the insulation while exhibiting high mechanical strength and high resistance to temperature cycling and mechanical impact, as well as good electrical characteristics. The invention also relates to a process for producing such elongated metal cylinder.
Insulated metal cylinders having an insulator formed on the outer surface of a metal pipe or rod are used as fasteners for contacts in circuit breakers. Those which are used at relatively low temperatures (about 100.degree. C.) commonly use organic insulators, and in some cases the insulator is made of a rolled sheeting of an organic material that is bonded to mica flakes with an organic adhesive.
Modern chemical plants, especially petrochemical complexes, use many gas or fluid conveying pipes having service temperatures as high as 200.degree. to 300.degree. C. The recent tendency is to replace these pipes by elongated insulated pipes having an insulator coat on the outer surface. Insulators made of organic materials will spall or peel entirely if they are subjected to elevated temperatures. This is an unavoidable physical phenomenon resulting from thermal expansion mismatching between the insulator and the metal pipe, and hence the organic insulator is entirely unsuitable for use under such elevated temperatures. A tubular insulator made of inorganic porcelain cannot be firmly fixed to the metal pipe so as to provide the necessary mechanical impact strength. A composite material based on asbestos containing an inorganic binder such as aluminum phosphate has a certain degree of mechanical strength and maintains fairly high electrical insulating properties at high temperatures. However, because of its inherent porous nature, this composite material has an unavoidable fatal defect in that its insulating properties drop suddenly if it is exposed to humid conditions at room temperature. Therefore, none of the insulating material available today exhibit completely satisfactory characteristics.
The present inventors previously proposed an insulator made of a glass-mica molded body. This insulator does not spall or drop if it is subjected to a temperature of about 300.degree. C. In addition, it retains high mechanical strength in the range of room temperature up to 300.degree. C., and exhibits high resistance to cold or heat and mechanical strength in the range of room temperature up to 300.degree. C., and exhibits high resistance to cold or heat and mechanical impact while maintaining good electrical characteristics. Furthermore, the characteristics of this insulator are not deteriorated even if it is subjected to temperature cycling. In spite of these excellent properties, the insulator has one serious problem concerning its manufacture: a long unit of the insulator is not obtainable.
In order to facilitate a better understanding of the features of the present invention, the characteristics of a glass-mica molded body and the conventional process for producing an elongated metal cylinder will be described.
The characteristics of the glass-mica molded body are governed to a great extent by the characteristics of the glass used. A glass-mica molded body using a glassy material having a transition point of about 400.degree. C. will not deform by softening even if it is subjected to a temperature of about 300.degree. C., and retains mechanical strengths comparable to that exhibited at room temperature. The electrical characteristics of the glass-mica molded body depend greatly on its composition; unless it contains an extremely great amount of an alkali metal oxide, the characteristics of the glass-mica molded body will not deteriorate appreciably even at 300.degree. C. and the necessary insulating properties can easily be ensured. Particularly good characteristics are exhibited by a glass mica molded body having lead oxide or zinc oxide as the principal base component, and boric acid or silicic acid as the principal acid component.
As regards the mica powder that is usable in preparing the intended glass-mica molded body, natural mica is not recommendable since, when heated in mixture with a glass powder, it reacts with the glass and is decomposed by losing the water of crystallization at a temperature lower than when it is heated independently. Synthetic mica having no water of crystallization is free from this tendency, and hence its powder is ideal for use in making the intended glass-mica is particularly advantageous.
A conventional insulated metal cylinder having an insulator coat made of the glass-mica molded body described above is shown in FIGS. 3A and 3B. FIG. 3A depicts an insulated rod having an insulator coat 201 of a glass-mica molded body formed around a metal rod 202, and FIG. 3B shows an insulated pipe having the same insulator coat 201 formed around a metal pipe 203. Both the metal rod 202 and the metal pipe 203 should preferably retain adequate mechanical strength and a thermal expansion coefficient of 8 to 11.times.10.sup.-6 under heating to a temperature between 500.degree. and 600.degree. C., and they are advantageously made of a steel material.
An example of the conventional method for producing an insulated pipe having a metal pipe 203 in the center will be described with reference to FIGS. 4A and 4B. This method uses a shaping mold consisting of four elements, a frame 204, a housing 205 of a split type having a feed filling cavity 205-1 on the top, a support 206 having a projection 206-1 in the center for fixing the metal pipe 203, and a plunger 207.
The feed is prepared from a mixture of 35 vol % of a glass powder (size: 200 mesh, transition point: 420.degree. C.) having a composition of 1.0 mole of B.sub.2 O.sub.3, 1.2 moles of SiO.sub.2, and 65 vol % of a synthetic fluorine-containing gold mica powder (size: 60 to 100 mesh). The mixed powder is wetted by addition of about 5 wt % of water, and the blend is cold-shaped with a press (not shown) into a cylindrical form that can be charged into the cavity 205-1. The cylinder is dehydrated to form a compact 208. As shown in FIGS. 4A and 4B, the top of the center through-hole in the insulated pipe 203 is sealed.
The shaping with this mold proceeds as follows. The frame 204, housing 205 and support 206 are assembled as shown in FIG. 4A, and the plunger 207 is left free. The mold is heated to 500.degree. C., the metal pipe 203 to 600.degree. C., and the compact 208 to 800.degree. C. After completion of the heating, the metal pipe 203 is placed on the support 206 within the housing 205, and the compact 208 is then charged into the cavity 205-1, as shown in FIG. 4A. Subsequently, the plunger 207 is placed on the compact 208 and urged with a press (not shown) against the compact 208 so that an insulator coat 201 is formed by forcing the compact 208 into a space 209 defined by the housing 205 and the metal pipe 203, as shown in FIG. 4B. The insulator coat 201 is cooled to 400.degree. C. (lower than the glass transition point) and the mold is disassembled to recover the shaped article, which is mechanically worked to provide an insulated pipe which, as shown in FIG. 4B, has a cylindrical insulator coat 201.
A short insulated rod or pipe that is produced by the method described above exhibits highly preferred characteristics since the insulator coat at position 201-1 near the area of contact with the plunger has a density close to that of the insulator coat at position 201-2, which is the farthest from position 201-1. However, as already mentioned, the conventional method has a fatal problem in that it cannot be used to fabricate a long insulated metal cylinder having the desired characteristics. The reasons are as follows: The mixture of glass and mica powders from which the insulator coat is made remains highly viscous even if it is heated. The viscosity of this mixture is highly dependent on the temperature so that it decreases with increasing temperature. A lower viscosity prevails if the compact 208 is heated to a higher temperature during molding, but the higher the temperature, the faster the rate of erosion of the mica by the glass. As a natural consequence, the temperature of heating the glass-mica blend is limited to a maximum of 800.degree. to 850.degree. C. From a strength viewpoint, the shaping mold cannot be heated to a temperature higher than 500.degree. C. During the shaping process, the compact 208 pressurized by the plunger 207 flows into the space 209, but when a temperature drop occurs as a result of contact with the inner wall of the housing, the viscosity of the molten compact increases rapidly and it no longer flows smoothly. As the length of the insulating area is increased, the space at position 201-2 is not completely filled with the molten compact 208 to provide a high density. This is why a long insulator coat 201 having uniform density cannot be formed.
This phenomenon is unavoidable and explains why a long insulated pipe having the desired characteristics cannot be produced by the prior art technique.
Petrochemical complexes and other chemical plants handle gases or liquids that show little corrosive effects on metals at room temperature but which become severely corrosive at elevated temperatures. At room temperatures, such gases or liquids can be conveyed through metal pipes. The transport efficiency of such liquids or gases is appreciably increased, however, if their temperature is increased to 200.degree. to 300.degree. C. at several points of the transport path. In this case though, if hot and, therefore corrosive, gases or liquids are conveyed, metal pipes having outer insulation coats must be employed at many points of the transportation circuit for safety reasons. In order to meet this requirement and secure an adequate mechanical strength, insulated and corrosion-resistant pipes composed of a metal pipe having a corrosion-resistant layer on the inner surface and an insulating layer on the outer surface can be used. (This type of metal pipes is hereunder referred to simply as corrosion-resistant pipes.) Many studies have been made regarding the fabricating of such corrosion-resistant pipes.
Among the pipes that have been previously proposed are metal pipes having a coat of a heat-resistant organic material formed on both inner and outer surfaces. Teflon and PEEK resins are organic materials having very high resistance to heat and corrosion. However, because of the inherent thermal expansion mismatching with the metal pipe, the organic coat expands at elevated temperatures and may spall in an extreme case. As a guide, organic materials have thermal expansion coefficients five to 10 times as great as that of a steel pipe. Because of this fatal defect, heat-resistant organic materials are not suitable for use in the manufacture of corrosion-resistant pipes having high service temperatures.
The use of inorganic materials has also been considered, and steel pipes with an enamel coat show great promise. The glaze used in enamelling steel pipes must have a thermal expansion coefficient between 10.5 and 12.0.times.10.sup.-6. This means a suitable glaze must have high concentrations of oxides of alkali metals such as lithium, potassium and sodium. The resulting enamel, often used for coating tableware, exhibits satisfactory resistance to the corrosive action of water having a temperature up to 100.degree. C. However, if the temperature of the water exceeds 100.degree. C. and if it is acidic, the corrosion resistance of the enamel coat suddenly drops to a practically unusable level.
Therefore, none of the materials so far proposed for use in the production of corrosion-resistant pipes has proved practically usable.
On the other hand, corrosion-resistant pipes having a coat of glass-mica molded body formed on both inner and outer surfaces have neither deformation nor spalling problems even at elevated temperatures between 200.degree. and 300.degree. C. In addition, a glass-mica molded body containing 50 to 70 vol % of a mica powder has a very high corrosion resistance, and therefore it exhibits excellent resistance to hot water, acids and alkalies, as well as good electrically insulating characteristics. Additionally, a thick and gas-permeable coat can be made from the glass-mica molded body. Therefore, the glass-mica molded body is considered to be ideal for use as a coating material for the corrosion-resistant pipe described above.
A problem, however, is that a long, corrosion-resistant pipe having a coat of glass-mica molded body cannot be produced by the conventional fabrication method.
The characteristics of the glass-mica molded body and the conventional process for fabricating a corrosion-resistant pipe with this body will hereunder be described. As mentioned above, the characteristics of the glass-mica molded body are governed to a great extent by the characteristics of the glass used in the molded body. A glass-mica molded body using a glassy material having a transition point of about 400.degree. C. will not deform even if it is subjected to a temperature of about 300.degree. C. Additionally, the electrical properties and mechanical strength of such glass-mica molded body are little different from those exhibited at room temperature. The thermal expansion coefficient of the glass-mica molded body is also highly dependent on the characteristics of the glass, and by changing the latter, glass-mica molded bodies having thermal expansion coefficients in the range of 8 to 11.times.10.sup.-6 can be obtained. The close relationship between the glass and the glass-mica molded body also applies to the corrosion-resisting properties, and a glass-mica molded body having improved corrosion resistance can be prepared using a highly corrosion-resistant glass.
Regarding the mica that is usable in preparing the intended glass-mica molded body, natural mica is not recommendable since it has a low pyrolytic temperature due to the presence of water of crystallization and because it is available in such various grades that products having consistent characteristics are hard to obtain. On the other hand, synthetic mica has a high thermal decomposition temperature and it is easy to obtain products having a consistent quality. Therefore, synthetic mica is exclusively used in the glass-mica molded body of interest. A synthetic fluorine-containing gold mica is particularly advantageous.
A corrosion-resistant pipe having a coat of the glass-mica molded body that is formed on both inner and outer surfaces by the conventional method will now described by reference to FIG. 5, wherein the corrosion-resistant pipe generally indicated at A is composed of a metal pipe 301 covered with an inner coat 302 and an outer coat 303.
The conventional method for producing such corrosion-resistant pipe is next described by reference to FIGS. 6A and 6B. This pipe is fabricated with a shaping mold. The mold consists of four components, a frame 304, a splittable housing 306 with a feed filling cavity 305 in the top, a support 307 having a projection 307-1 for retaining an insert 309 and the metal pipe 301 in the central position, and a plunger 308.
The glass in the feed has, for instance, a composition of 70 wt % PbO, 16 wt % B.sub.2 O.sub.3 and 14 wt % SiO.sub.2, a transition point of 400.degree. C., and is used after being ground to a size of 200 mesh. The mica in the feed is a powder of synthetic fluorine-containing mica having a grain size of 60 to 100 mesh. Equal weights of the glass and mica powders are mixed to prepare the feed powder, which is set by addition of about 5 wt% of water. The blend is cold shaped with a press (not shown) into a cylindrical form that can be charged into the cavity 305. The cylinder is dewatered to form a compact 310.
The shaping with this mold proceeds as follows: The frame 304, housing 306 and support 307 are assembled as shown in FIG. 6A, and the plunger 308 is left free. The mold is heated to 550.degree. C., the insert 309 and metal pipe 301 to 600.degree. C., and the compact 310 to 800.degree. C. After completion of the heating, the insert 309 and metal pipe 301 are placed on the support 307 within the housing 306, and the compact 310 is then charged into the cavity 305, as shown in FIG. 6A. Subsequently, the plunger 308 is placed on the compact 310 and urged with a press (not shown) against the compact 310 so that the latter is forced into a space 311 that is defined by the metal pipe 301 and insert 309, as well as into a space 312 defined by the metal pipe 301 and the housing 306, thereby forming an inner coat 302 and an outer coat 303, as shown in FIG. 6B. These coats 302 and 302 are cooled at 380.degree. C. (lower than the glass transition point) and the mold is disassembled to recover the shaped article, which is mechanically worked to cut off the insert 309 and provide the corrosion-resistant pipe A as shown in FIG. 5.
The corrosion-resistant pipe A fabricated by the conventional method described above possess ideal characteristics if its length is small, but a fatal problem is that a long pipe having the desired characteristics cannot be obtained. The reasons are as follows: The mixture of glass and mica powders from which the corrosion-resistant coat is made remains highly viscous even if it is heated. The viscosity of this mixture is highly dependent on temperature so that it decreases with increasing temperature and increases rapidly with the decreasing temperature. A lower viscosity prevails if the compact 310 is heated to a higher temperature during molding, but the higher the temperature, the faster the rate of erosion of the mica by the glass. As a natural consequence, the temperature of heating the glass-mica blend is limited to a maximum of 800.degree. to 850.degree. C. The temperature of the shaping mold is also related to the mechanical strength, and it cannot be heated to a temperature higher than 550.degree. C. During the shaping process, the compact 310 pressurized by the plunger 308 flows into the spaces 311 and 312, but the temperature of the compact 310 drops since its front is flowing in contact with the insert 309, metal pipe 301 and the inner wall of the housing 306. As a result of this temperature drop, the viscosity of the compact 310 increases rapidly and it no longer flows smoothly. As the length of corrosion-resistant pipe is increased, the bottom portions 311-1 and 312-1 of the spaces 311 and 312, respectively, are not completely filled with the molten compact 310 to provide a high density. For this reason, a long corrosion-resistant pipe having a uniform inner coat 302 or outer coat 303 cannot be produced.
This phenomenon is unavoidable in the conventional process and explains why a long corrosion-resistant pipe. The prior art process also requires the step of cutting off the insert 309 from the shaped article by mechanical working, but this step is quite time-consuming and leads to a high price of the final product.
Moreover, the casing (1, 11) used under the conditions discussed above must meet strict requirements. The first requirement to be satisfied is high mechanical strength. Since the casing must be strong enough to withstand the internal pressure and the pulling action of a suspended object, the inevitable choice is a metal pipe. Secondly, the casing must have good corrosion resistance. However, the life of a metal pipe is quite short under the expected severely corrosive environment where the casing is subjected to heated steam (300.degree. to 320.degree. C.) in the presence of sodium chloride or hydrogen sulfide. Thirdly, the casing must be airtight in order to avoid any leakage of the oil into a geological formation above the oil-sand stratum. The part of the casing buried in the oil-sand formation must have a particularly great corrosion resistance, but the requirements for the part of the casing in the overlying geological formation are far less stringent.
The choice of the material for the part of the casing to be buried in oil-sand deposits is quite limited, and the practically feasible casing is a corrosion-resistant pipe having a coat of a corrosion-resistant material formed on both inner and outer surfaces of a base metal pipe.
Corrosion-resistant coats formed on the metal pipes are commonly made of PEEK resins or Teflon resins. These resins, when used alone, exhibit very good heat- and corrosion-resistant characteristics, and corrosion-resistant pipes having coats of such resins on both inner and outer surfaces of a metal pipe can be used very effectively if their service temperature is in the range of from room temperature up to 100.degree. C. However, this is not the case for the temperature range of 300.degree. to 320.degree. C. to which the actual casing is subjected. PEEK resins or Teflon resins, as mentioned previously, have thermal expansion coefficients five to 10 times as great as that of the metal pipe, and as the temperature rises, the coat made of such resins deform greatly and may either spall or break in an extreme case. This phenomenon is highly likely to occur when the coat is subjected to temperature cycling, and hence the resins mentioned above are entirely unsuitable for use in making the intended corrosion-resistant pipe.
On the other hand, the glass-mica molded body has a thermal expansion coefficient that matches well with that of the metal pipe, and so, a corrosion-resistant pipe having a coat of this molded body formed on the metal pipe will not suffer from spalling, breaking or peeling of the coat even if the pipe is used at 300.degree. to 320.degree. C. or subjected to temperature cycling within this range. In addition, the coat of glass-mica molded body exhibits excellent resistance to hot water, salt water or H.sub.2 S-containing water that has a temperature of about 300.degree. C. Therefore, a metal pipe having the coat of glass-mica molded body is considered to be ideal for use as a corrosion-resistant pipe, but, as explained above, since a long unit of the pipe cannot be fabricated, this pipe is not suitable for use as a casing through which high-pressure steam is forced to recover the oil-sand deposits.
In geothermal power generation, hot water having higher temperatures than that obtainable from the existing hot springs is used. This hot water is usually pumped and conveyed through steel pipes, but the metal pipes tend to rapidly corrode, and further their life is shortened if the hot water is acidic. From an economical viewpoint, there is a rapidly growing need for the use of pipes having improved corrosion resistance. The prerequisite for these pipes is that they have high mechanical strength and that the inner surface of the pipe have a particularly great corrosion resistance. Pipes made of organic materials and having good corrosion resistance are available in many types of commercial products, but because of their low mechanical strength, such organic pipes cannot be used independently for conveying hot fluids. Metal pipes having an inner coat of an organic material exhibit very good characteristics under room temperature conditions, but not at elevated temperatures. A glass-mica molded body inner layer is thus preferred. Such a glass-mica molded body also exhibits good electrical insulating properties and hence may be effectively used in electrical insulator tubes.
A pipe having an inner coat of a glass-mica molded body formed on the inner surface by a conventional method is now described by reference to FIG. 7A, wherein the pipe generally indicated at 500 consists of a metal pipe 501 having a coat 502 on the inner surface.
The conventional method for producing such pipe will next be described with reference to FIGS. 7B and 7C. This pipe is fabricated with a shaping mold. The mold is composed of four components, a frame 504, a splittable housing 506 with a feed filling cavity 505 in the top, a support 507 having a projection 507-1 for retaining an insert 509 in the central position, and a plunger 508.
The plunger 508 is placed on a compact 510 and urged with a press (not shown) against the compact 510 so that the latter is forced into a space 511 defined by the metal pipe 501 and the insert 509, thereby forming the coat 502 as shown in FIG. 7C. The coat 502 is cooled to 380.degree. C. (lower than the glass transition point) and the shaping mold is then disassembled to recover the shaped article, which is mechanically worked to cut off the insert 509 and provide the coated pipe 501 as shown in FIG. 7A.
During the shaping process, the compact 510, pressurized by the plunger 508, flows into the space 511, but when the temperature of the compact 510 is reduced as a result of contact with the insert 509 and metal pipe 501, the viscosity of the compact increases rapidly and it no longer flows smoothly. As the length of the coated pipe is increased, the front portion 502-1 in FIG. 7B and 7C is not completely filled with the compact 510 to provide a high density. This is why a long pipe having a uniform coat 502 cannot be produced.